Transparency in PDF has been around since 2001, but still seems to be one of those technologies that people are uncomfortable using. To a lot of people (graphic designers included) transparency in PDF and the white lines it tends to generate on the monitor are a mystery, so we often get support questions and see forum posts about transparency issues. Allow us to give you chapter and verse on transparency, and on white lines in particular.
Why use transparency in PDFs?
Transparency in PDFs is popularly used for special effects. Transparent objects are image or text that appear to show through the page, as if they’re ‘hiding’ behind a layer of fog or tinted glass. Among other effects, transparency is also often used to create drop shadows.
What you see is not necessarily what you get
Transparency functionality first appeared in PDF 1.4, the specification of which was published in 2001. It took a little while to reach desktop applications, but when it did, it caused all sorts of issues. Mainly because most output devices at the time had RIPs that were postscript based and did not support transparency flattening needed to output files with transparent objects correctly.
Transparency flattening in PDFs
Transparency flattening means removing or calculating the transparency effects so that the finished job will look as it does on the monitor. When a PDF file is flattened, all elements in transparent areas are rasterized to images. The balance between raster and vector is controlled by the slider option you can see in the screenshot. Typically, you want this to be at 100 on the vector side, to minimize the amount of rasterization.
What’s the deal with those white lines in your PDF?
One of the side effects of transparency flattening is that the created images are split into slices or ‘atomic regions’ which are indicated by very thin white lines.
Seeing these thin white lines on screen can be scary to the uninitiated, as they give the impression they will print — and sometimes they do! That’s when PitStop 13 (update 2) comes into play. Thanks to the ‘turn off anti-aliasing when rasterizing’ option, white lines from flattening are no longer an issue.
Curious to see how it works? This video shows you all you need to know:
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